Across Britain, people keep fighting the same patches spreading on their walls.
Homeowners now turn to a 19th‑century lime blend shared across families and farmhouses. The recipe uses four simple ingredients and a stiff brush. It aims to let walls breathe, not trap moisture behind paint. Interest grows as energy bills rise and windows stay shut for longer.
The story and why it matters now
Older homes handled moisture with breathable materials. Lime plasters drew vapour through the wall and let it offload gently. Modern impermeable paints and vinyl wallpapers changed that. They seal the skin of a wall and lock damp behind a glossy film. Stains bloom, paint blisters, and mould finds a foothold.
The revived mix pairs slaked lime, fine silica sand, a pinch of borax and a little oil. It creates a thin, mineral skin that sheds surface water while allowing water vapour to travel out. The high pH in lime and the borate salt raise an unfriendly setting for mould and mildew.
A wall that breathes sheds moisture; a sealed wall hoards it. Breathability beats brute waterproofing in many old buildings.
How the four ingredients pull their weight
- Slaked lime gives alkalinity and breathability. It buffers moisture swings and hinders mould growth.
- Fine silica sand adds structure so the coat grips and dries evenly.
- Borax acts as a mild fungistat. It helps keep regrowth in check on mineral surfaces.
- Olive oil or boiled linseed oil adds water repellence without forming a plastic film.
| Ingredient | Quantity | Role | Typical UK price |
|---|---|---|---|
| Slaked lime (hydrated) | 2 kg | Breathable base, high pH | £6–£10 |
| Fine silica sand (≤0.5 mm) | 3 kg | Body and grip | £3–£5 |
| Borax powder | 250 g | Fungistatic support | £3–£6 |
| Olive oil (first press) or boiled linseed oil | 500 ml olive or 300 ml linseed | Light water repellence | £4–£8 |
Step‑by‑step: from bucket to wall
Allow a minimum of 48 hours of natural drying for the first change: a fresher smell and stains that start to pale.
What you can expect and when
Within two days, that stale, earthy odour often fades. The wall surface feels drier to the touch. The grey or brown halo around old marks turns lighter. Over the first week, condensation beads less on the treated zone. After three to four weeks, many homes see new stains slow or stop, provided the source of moisture stays moderate and rooms get regular air changes.
Plan to refresh the coat every 7–10 years on exposed walls. Kitchens and bathrooms may need attention sooner. Keep notes on where you applied the mix and the date. Photos taken on day one, day two and week four help you track progress.
Safety, suitability and risks
- Wear gloves and a dust mask when handling lime and borax. Avoid skin and eye contact.
- Keep the mix away from food prep areas until it cures. Store powders out of reach of children and pets.
- Use on mineral backgrounds: brick, stone, lime render, cement render. Avoid plasterboard and gypsum skim, which soften with persistent damp.
- If paint refuses to key, you likely have an impermeable film. Strip that layer or use a breathable remover first.
- Persistent tide marks above skirting boards point to rising damp and salts. The mix can help the surface, yet you still need to manage ground moisture and salts.
- Heavy leaks, rotten timber, or black mould across large areas call for a survey. Deal with sources before cosmetic fixes.
Ventilation and habits that boost results
Moisture in homes comes from showers, cooking, drying clothes and even breathing. You can cut peaks with small daily changes.
- Open opposing windows for 5–10 minutes twice a day to purge humid air.
- Use lids on pans and run extractor fans during and 15 minutes after cooking or showering.
- Keep furniture 5–10 cm off cold exterior walls to let air move.
- Dry clothes outside or in a vented room with the door shut and a fan on.
- Maintain steady low heat in winter to avoid dew point swings on cold corners.
A quick guide to ratios, coverage and timing
Mix by weight: 2 parts slaked lime, 3 parts fine sand, 0.125 part borax, and about 0.25 part oil (olive) or 0.15 part boiled linseed. One batch covers roughly 6–8 m² as a thin brush coat on a smooth wall and less on rough masonry. Work in spring or a dry spell. Skip humid or rainy days. Keep windows cracked for airflow while the coat cures.
Patch test before you commit
Pick a discreet 30 × 30 cm spot. Apply the mix. Let it cure 48 hours. Check for adhesion and any colour shift. If it powders off, your base still holds contaminants. Clean deeper and try again. If it bonds, proceed to the wider area in small sections.
What makes it different from a waterproof paint
Waterproof paints trap liquid water behind a non‑breathable film. They often look fine for months, then lift in blisters. The lime‑based coat trades outright blocking for controlled vapour movement. It reduces surface wetting while letting the wall purge moisture from within. You see fewer cold‑spot condensing circles and less stress on the substrate.
Extra help if you rent or need a reversible fix
You can still benefit without reworking the wall. Use a portable dehumidifier to keep indoor relative humidity near 50–55%. Fit a draught‑proof but trickle‑vented window latch if your landlord agrees. Aim furniture away from cold corners. If you gain permission later, a breathable coat on problem areas can pair well with a dehumidifier for a belt‑and‑braces approach.
When numbers help your decision
A cheap digital hygrometer tells you the room’s relative humidity. Track morning and evening values for a week. If readings sit above 65% for long periods, you will fight condensation no matter what is on the wall. Reduce sources first, then apply the mix. If values drop to near 55% and stains still spread, look for leaks or bridging at the base of the wall.
Final pointers that raise your odds
- Target no more than 2 m² at a time so the coat stays workable and even.
- Brush into mortar joints and edges where moisture often lingers.
- Do not gloss over with vinyl paint. Choose lime wash or other breathable finishes after curing.
- Re‑check the spot after rain and after a long cooking session to see how it copes with spikes.
Small, regular actions stack up: breathable materials, smart ventilation and steady heat bring most homes under control within a month.
You can swap olive oil for boiled linseed oil at the lower dose shown in the table. The linseed version tends to set a touch faster and feel slightly harder under the brush. Both options aim for the same end: create a surface that sheds splashes while letting vapour escape. Match the mix to your wall, keep a log, and adjust your routine. The 48‑hour turnaround on smell and colour often arrives when the rest of the house plays its part.
